For the treatment of web-like media, such as e.g. paper, by pressing, drying and smoothing, rolls are used which have large diameters and are heated. In this context "large" is appreciated to be a diameter of at least 1,000 mm.
These involve e.g. the so-called "center rolls" of a press section of a paper machine, the usual drying cylinders in the drying section and particularly the heated rolls in smoothing sections, the so-called "gloss cylinders" and "soft calenders". The so-called "Yankee cylinder" represents a special case, a combination of drying cylinder and smoothing roll.
Heating such rolls is done preferably from within by means of heat transfer fluids, preferably water, steam or heat transfer oil. The increase in the surface temperature of the rolls achieved thereby permits either their function, by affecting in the case of drying cylinders the evaporation of the moisture in the paper web, or promotes their function, because the water in the press can be more easily squeezed out from the paper web at elevated temperature and reduced viscosity or because the plastification ability of the paper fibers is enhanced at elevated temperature so that they can be smoothed more easily.
When heating is done by means of steam, the rolls are normally of the cylindrical kind. The steam is usally introduced into the interior of the roll cylinder where it condenses on the inside surfaces. The condensate is drawn off from the roll via a syphon.
The drawback in this arrangement is that in such a configuration the complete interior space is subjected to steam pressure. This results in restrictions mainly stemming from safety considerations. These relate to the level of the permissible steam pressure and thus to the achievable surface temperature which, in turn, restricts the effectiveness of the process. Restrictions also exist--depending on where employed--as regards the application of roll materials. Since, for instance in U.S.A. the so-called chilled cast iron, as is usually employed for a smoothing roll, is not standardized, it is out of the question for such applications. Large rolls for gloss calenders thus need to be manufactured from gray cast iron or nodular cast iron which as regards the wearing properties of the roll surface is unfavorable.
Due to the mechanical stresses the wall of the roll cannot be designed as thin as would be desirable for a good conduction of the thermal energy to the heat transfer medium.
When certain rotative speeds are attained, a stable ring of condensate is formed on the inside of the roll due to the centrigugal effect and dependent on the roll diameter. Then, the steam no longer condenses on the wall of the roll, but in the condensate ring. This results in a deterioration, on the one hand, of the heat transfer from the steam to the roll, but also, on the other, of the temperature profile at the roll surface.
Some of the cited drawbacks, such as e.g. the poor temperature profile, were overcome in the case of cylindrical rolls by changing to hot water for the heating. For this purpose a cylindrical displacement body was secured in the central bore of the roll so that a narrow, ring-shaped gap remained between the displacement body and the inner bore. The hot water was then forced through this gap at velocities exceeding 1 m/s.
The drawbacks of the large wall thickness, required for mechanical reasons, and the restriction due to the internal pressures remained, however, and apart from this such rolls are of a heavy-weight structure.
The aforementioned drawbacks were overcome in part by the so-called peripherally drilled rolls. In this case the heat transfer fluid--water or heat transfer oil--flows through axially parallel drilled passages located just beneath the surface of the roll. This enabled the distance for conducting the heat from the heat transfer medium to the surface of the roll to be decisively reduced.
Unfortunately, this design too has several drawbacks. For safety reasons water is used only up to a water temperature of max. 170.degree. C. In addition to this, heat transfer oil is used. The latter is accepted by paper mills merely as a necessary evil. Even minor leakage in pumps, sealing heads or in the mechanically highly stressed rolls are a considerable nuisance. Handling the oil by pump transfer or other procedures always involves a possible hazard for the environment.
The volume of liquid needing to be pumped is also a problem in the case of large rolls. In flowing through the roll the heat transfer medium gives off part of its heat to the roll, it thereby losing temperature. For reasons of an homogenous surface temperature only a limited drop in temperature of the heat transfer medium is permissible. This is the reason why--depending on the heating capacity of the roll--the liquid throughput needs to be selected correspondingly high. The higher this is selected, the lower is the drop in temperature.
For a heated "center roll" roughly 8 m wide a heating capacity of roughly 1,800 kW needs to be assumed according to the publication of one paper machine manufacturer. If such a roll is heated with water, then a throughput of 72 liters/sec. is necessary for a permissible drop in temperature of the water of 6.degree. C. This is still 43 liters/sec. for 10.degree. C.
If such a roll is heated with heat transfer oil, then the throughput increases to roughly twice as much due to the lower specific heat of this heat transfer medium.
It has now been proposed within the scope of the German utility model No. G 93 06 176.5 in the same field to also employ steam as the heat transfer medium in peripherally drilled rolls to thus get round the drawbacks of the water or the heat transfer oil. However, this design poses some problems in the application to large steam-heated rolls in accordance with the proposed principle.
In this case, for instance, it is provided for that the heating steam is introduced from both ends of the roll into the peripheral drilled passages. For this purpose, part of the steam entering through the sealing head into the pin at the heating end is directed through the central bore in the roll body to the other end of the roll and from there into the associated ends of the peripheral drilled passages. Particularly due to the pressure vessel code requirements, this central bore is produced with a diameter that is smaller than 150 min. The rolls concerned then fail to fall under the pertinent code requirements in countries in which the US ASME code requirements apply.
In the case of large rolls, however, this would mean that these rolls need to be designed almost solid. The savings in weight as are possible with an enlarged central bore thus fail to be achieved.
Although it would be possible to incorporate in an enlarged central bore a tube for handling the steam which has an internal diameter that is smaller than 150 mm, the central support of such a tube, taking into account the thermal expansion and vibration in operation, is highly complicated, however.
Also, the distribution of the heating steam, as proposed in this utility model, fails to be achievable in this way when large rolls are concerned. Together with the diameter the number of peripheral drilled passages increases to such an extent that they can no longer be connected individually to the central bore in the pin. Providing any kind of receiver spaces in the pins or in the roll body or between the two is also not allowed, for then the prerequisite for the ASME Code (diameter&lt;150 mm) failing to apply would no longer be satisfied. But even if this were of no importance, receiver spaces, from which the connection to the peripheral drilled passages could be made, would have to be closed off by heavy pressure caps from the interior of the roll, when the latter itself is required to remain pressureless.
New considerations also need to be made for discharging the condensate to be stripped from the peripheral drilled passages of the roll. In case that a syphon becomes completely filled with condensate, the overpressure necessary to strip this syphon is calculated from the resistance of a condensate column from the outer edge of the roll to its center as dictated by the centrifugal force. When such an overpressure is applied, then large quantities of stripping steam will blow through from all other syphons with the corresponding losses. Apart from this, achieving the connection of the individual syphons to the central condensate receiver poses difficulties in design.
In conclusion, returning the resulting condensate to the drive end by a central tube requires a highly complicated design, especially when the latter itself needs to be accommodated in a central steam tube (see above).